As well known, polyvinyl chloride and paper have been the primary material used in manufacturing various magnetic cards, e.g., telephone cards, credit cards and subway tickets. However, a magnetic card prepared from polyvinyl chloride is often felt rather inconvenient due to its considerable thickness, creating a nuisance to a user carrying several of such cards; is liable to be damaged due to its low levels of impact strength, flexibility and heat resistance; and may be deformed by heat generated during the printing process. In case that a magnetic card is prepared from paper, there also exist various problems stemming from its poor water resistance, mechanical strength, durability and the like.
Because of these deficiencies, therefore, there have been made studies for the development of a polyester film having high opacity and surface gloss suitable for the manufacture of magnetic cards. In this connection, Japanese Patent Laid-open Publication Nos. 88-161029 and 88-137927 describe the treatment of a polyester film with a particulate calcium carbonate compound; and Japanese Patent Laid-open Publication No. 87-241928 suggests the employment of TiO.sub.2 and SiO.sub.2 in a polyester film. Also, Japanese Patent Laid-open Publication Nos. 91-50241 and 90-185532 disclose that addition of calcium carbonate and barium sulfate to a polyester film can provide the film with improved concealability and whiteness. Further, Japanese Patent Laid-open Publication No. 88-193934 employs a titanium compound in the form of anatase together with zinc.
However, these prior art methods and compositions for producing polyester films also have a number of deficiencies. For example, titanium compounds tend to discolor the polyester film upon its exposure to a high temperature or sun light; and have the tendency to agglomerate due to the poor dispersibility thereof during the manufacture of the polyester film, rendering it difficult to prepare the polyester film with a uniform thickness and good surface property. Also, these polyester films are likely to generate static electricity, causing such problems as non-uniform printing and poor receptivity to ink during the printing process and even loss of information recorded on the magnetic cards.